Since polyimides have a large number of higher-order structures composed of cyclic structures such as a heterocyclic ring and aromatic ring, their molecular chains are hard to move at high temperatures. Due to a large number of higher-order bonds such as double bonds, they have large interatomic bonding energy. The heterocyclic ring and aromatic ring in a polyimide molecule interact with one another within the polymer molecule or between the polymer molecules to form a charge transfer complex, thereby to produce a large coagulation power. Therefore, they have the highest heat resistance in various plastics.
Furthermore, polyimides are excellent not only in heat resistance but also in mechanical properties and electric properties. Specifically, polyimides have high strength, high elasticity, high insulating properties and low dielectric properties. Polyimides are also excellent in chemical resistance, radiation resistance and flame retardance.
A photosensitive polyimide which has been developed recently has been used for a very large-scale integration semiconductor, a polyimide having toughness and strong adhesive force has been used for a launch vehicle, a polyimide having high transparency has been used for an optical communication device, and a polyimide having good injection molding property has been used for heat resistant sliding parts including automobile parts.
Application examples of polyimides having the above-mentioned properties in optical fields are given below: a polyimide prepared by reacting a dianhydride containing a sulfur atom with a diamine containing no sulfur atom is used for a light guide (see Patent Document 1); a polyimide prepared by reacting a dianhydride containing no sulfur atom with a diamine containing a sulfur atom is used for an alignment film of liquid crystal (see Patent Document 2); a mixture of a polyimide having a particular structure and titanium oxide particles is used for a high-refractive index material (see Patent Document 3); and a mixture of a polyamic acid containing no sulfur atom, titanium oxide particles and other particular compound is used for a positive type photosensitive resin composition (see Patent Document 4).
[Patent Document 1] JP-A-2004-131684
[Patent Document 2] JP-A-H05-263077
[Patent Document 3] JP-A-2001-354853
[Patent Document 4] JP-A-2005-208465
In the case where a sulfur atom is introduced into the dianhydride (Patent Document 1), the resultant polyimide has a refractive index of 1.67 to 1.72, but is insufficient in transparency at visible light wavelength (400 to 700 nm). In the case where the polyimide is used for an alignment film of liquid crystal (Patent Document 2), since attention is focused on the conductivity of the polyimide, no reference is made to the refractive index. The invention disclosed in Patent Document 3 uses the polyamic acid prepared by reacting a specific dianhydride with a specific diamine. The diamine used in the invention disclosed in Patent Document 3 has a specific structure in which benzene rings are bonded mainly by an ether group (—O—), and Patent Document 3 discloses only two kinds of diamines having one thioether group (—S—).